The present invention relates to cryogenic refrigerators, in particular pulse tube refrigerators, Gifford McMahon (GM) refrigerators, and Solvay refrigerators, that use air conditioning type oil lubricated compressors. These refrigerators consist of an expander, sometimes referred to as a coldhead, which is connected to a remote compressor by supply and return gas lines. Such systems are in widespread use in cooling MRI magnets and cryopumps. These refrigerators have expanders that run at low speed, relative to the compressor, and are thus able to produce refrigeration efficiently at temperatures in the range of 4 K to 20 K. Oil in the compressor is typically removed from the helium working fluid in an oil separator that returns oil to the compressor, followed by an adsorber that is replaced at intervals of about 18 months
While manufacturing procedures have been developed that assure that the helium is clean enough so that new refrigerators run at stable temperatures for a long period of time, it has been found that contaminants are sometimes introduced during service. This can cause the operating temperature to rise. When this happens it has been found to be very difficult to remove these contaminants in the field, and it is customary to replace the compressor to restore acceptable operation.
A method of returning a refrigerator to acceptable operation after it has become contaminated has been discovered and is the object of this invention.